Create advanced.md Client documentation file

[drbyte]

Added section on detecting Expose as the active proxy server. This can be useful when taking custom action based on the kind of proxy server being used (such as overriding non-standard routing logic, auto-selecting proxy-specific databases, etc)

Ref:

expose/app/Server/Http/Controllers/TunnelMessageController.php

Lines 106 to 124 in d98eabe

	protected function prepareRequest(Request $request, ControlConnection $controlConnection): Request
	{
	$request::setTrustedProxies([$controlConnection->socket->remoteAddress, '127.0.0.1'], Request::HEADER_X_FORWARDED_ALL);
	$host = $this->configuration->hostname();
	if (! $request->isSecure()) {
	$host .= ":{$this->configuration->port()}";
	}
	$request->headers->set('Host', $controlConnection->host);
	$request->headers->set('X-Forwarded-Proto', $request->isSecure() ? 'https' : 'http');
	$request->headers->set('X-Expose-Request-ID', uniqid());
	$request->headers->set('Upgrade-Insecure-Requests', 1);
	$request->headers->set('X-Exposed-By', config('app.name').' '.config('app.version'));
	$request->headers->set('X-Original-Host', "{$controlConnection->subdomain}.{$host}");
	return $request;
	}

[drbyte]

While I haven't studied it fully to be sure, can the client use a project-specific .expose.php file for config overrides, similar to how the Server can? If so, I envision info about that being added to this advanced.md doc as well ... hence creating the new file for this PR instead of combining it into someplace else.

[mpociot]

Yeah, that's right @drbyte. Expose will look in two different locations for the config file:

First in the current working directory as .expose.php and then in ~/.expose/config.php.
So yeah, project-specific expose configuration is possible - but yet undocumented.

I'll leave this PR open and would add the changes myself and then add this file to the navigation as well.